Technical Field
The present disclosure relates to a light emitting device and the manufacturing method thereof, in particular to a chip-scale packaging light emitting diode (LED) device, which includes a flip-chip LED semiconductor die generating optical radiation while in operation.
Descriptions of the Related Art
LEDs are widely used in various applications including traffic lights, backlight units, general lightings, portable devices, automotive lighting and so forth. Generally, an LED semiconductor die is disposed inside a package structure, such as a lead frame, to form a packaged LED device. It may further be disposed and covered by photoluminescent materials, such as phosphors, to form a phosphor-converted white LED device.
Recent development of a chip-scale packaging (CSP) LED device has attracted more and more attention due to its promising advantages. As a typical example, a white-light CSP LED device is generally composed of a blue-light LED semiconductor die and a photoluminescent structure covering the LED semiconductor die in a compact chip-scale size. The blue LED semiconductor die is typically a flip-chip LED die emitting blue light from an upper surface and four peripheral edge surfaces simultaneously. The photoluminescent structure is disposed covering the LED semiconductor die to down-convert the wavelength of the blue light emitted from the upper surface as well as the four peripheral edge surfaces. After passing through the photoluminescent structure, a portion of the blue light is converted into a higher wavelength (lower energy) light, and the light with different wavelength spectrum is subsequently mixed in a prescribed ratio to generate a desired color temperature white light. In order to achieve the purpose of converting blue light uniformly, it is desired that a photoluminescent structure has the same thickness and the same distribution density of a photoluminescent material on the upper surface and the four peripheral edge surfaces, namely forming a conformal coating layer of the photoluminescent structure.
In comparison with a plastic leaded chip carrier (PLCC) LED device, a CSP light emitting device shows the following advantages: (1) The material cost is considerably less by eliminating the use of a bonding wire and a lead frame. (2) The thermal resistance between a LED semiconductor die and a mounting substrate, typically a printed circuit board (PCB), is further reduced without using a lead frame in between. Therefore the LED operation temperature is lowered while under the same driving current. In other words, less electrical energy can be consumed to obtain more optical power for a CSP LED device. (3) A lower operation temperature provides a higher LED semiconductor quantum efficiency for a CSP LED device. (4) A much smaller form factor of the light source provides more design flexibility for module-level LED fixtures. (5) A CSP LED device having a small light emitting area more resembles a point source and thus makes the design of secondary optics easier. A compact CSP LED device can be designed to generate small-Etendue light with higher optical intensity that is specified for some projected light applications, such as automobile headlights.
Even though CSP LED devices have many advantages, however, the adhesion strength between a photoluminescent structure and an LED semiconductor die is relatively weak compared to that of PLCC type LED devices. For a CSP LED device, a lead frame or a submount may be omitted; thus the photoluminescent structure can primarily or solely make surface contact with the LED semiconductor die, without additional surface contact with a lead frame or submount. Therefore, the contact area of the photoluminescent structure is relatively reduced to just the LED semiconductor die surface without additional contact to the submount area. Reduced bonding area generally results in a poor bonding force between the photoluminescent structure and the LED semiconductor die. Further, the coefficient of thermal expansion (CTE) of a photoluminescent structure material (generally comprised of organic resin materials) is typically much larger than that of an LED semiconductor die material (generally comprised of inorganic materials). While operating a CSP LED device, a noticeable CTE mismatch during a thermal cycle will induce internal stress between the interfaces of these two materials. Because of the poor adhesion strength to the LED semiconductor die, the photoluminescent structure tends to be easily delaminated and peeled off from the LED semiconductor die. Delamination of the photoluminescent structure is one of the major CSP failure mechanisms during operation of a CSP-type LED device. This shortcoming impacts the reliability of a CSP LED device and poses practical constraints for CSP LED applications.
Another problem of a CSP-type LED device is poor color uniformity. There are two major mechanisms causing poor color uniformity: 1. Inconsistent mechanical dimensions of a photoluminescent structure, and 2. Uncontrollable phosphor particle distribution inside the photoluminescent structure from one device to another device. In a comparative manufacturing process to fabricate CSP-type LED devices, firstly, phosphor particles are mixed within a binder resin material to form a phosphor slurry; and subsequently, the phosphor slurry is disposed on an LED semiconductor die using methods including molding, screen printing, spraying or the like to form a photoluminescent structure. It will be appreciated that, when the phosphor slurry is used to form a photoluminescent structure, precise control of geometric dimensions of the photoluminescent structure is desired to achieve a desired correlated color temperature (CCT) of a CSP LED device. However, it is quite challenging to control the dimensions of each photoluminescent structure precisely and consistently in a mass production fabrication process. Even if geometric dimensions of a photoluminescent structure are accurately controlled during mass production, there is lack of a mechanism to control the particle distribution of phosphor materials inside the photoluminescent structure. In fact, distribution control of phosphor materials is one of the most important factors determining optical properties, such as CCT, of a CSP LED device. It is therefore highly desirable to achieve conformal coating of phosphor particles over an upper surface and four edge surfaces of an LED semiconductor die to form a photoluminescent structure, so that consistent optical conversion properties of CSP LED devices can be achieved in a mass production fabrication process.
Specifically, several manufacturing stages in a mass production process could result in poor color uniformity for CSP-type LED devices resulting in a low manufacturing yield. For example, if a photoluminescent structure of a CSP LED device is formed from a phosphor slurry through molding (or screen printing) process, a prerequisite stage is to arrange a plurality of LED semiconductor dies to form an array inside an inner surface of a mold (or a stencil). However, imprecise arrangement to form an array of LED semiconductor dies will create inconsistent thickness on a top side and edge sides of a photoluminescent structure. Besides, after an array of photoluminescent structures is formed, a singulation process is usually used subsequently to separate the array of CSP LED devices. Imperfect position control of a dicing saw will result in inconsistent thickness on the edge surfaces of a photoluminescent structure from one CSP LED device to another CSP LED device. Also, attaining color uniformity is impeded in that the particle distribution of phosphor materials inside the phosphor slurry could not be controlled. As a result, the light passing through inconsistent photoluminescent structures of LED devices will produce poor color uniformity. Even for an individual CSP LED device, poor spatial color uniformity over different viewing angles is not uncommon due to various phosphor conversion distances of a photoluminescent structure that blue light will experience. Poor color consistency causes a low manufacturing yield during mass production.
Alternatively, a spray coating process is another fabrication process to form a photoluminescent structure with a conformally coated phosphor layer. Issues caused by imprecise arrangement of LED semiconductor dies may be alleviated using a spraying process as compared to using molding or screen printing processes. However, other challenges occur if a photoluminescent structure is fabricated using a spraying process. It is found that it is difficult to retain phosphor particles next to vertical edge surfaces of the LED semiconductor dies because the gravity effect will cause phosphor particles precipitating during fabrication. This gravity effect will make it difficult to form a substantially continuous layer of the phosphor material on edge surfaces of an LED semiconductor die. Although a transparent and continuous resin layer can be formed on edge surfaces of an LED semiconductor die, the phosphor particle layer inside the resin material is locally discontinuous. In other words, a photoluminescent structure is fabricated using a spraying process will form substantially large and optically transparent structures of “voids”. Blue light irradiated from the LED semiconductor die therefore leaks more from the voids, causing the blue light to directly pass through with less chance of wavelength conversion by the phosphor materials. It will be appreciated that the voids typically found on the four vertical edge surfaces will result in blue light leaking more from the edge surfaces of a CSP LED device. A blue ring is therefore generated for such CSP LED devices fabricated using this spraying process. In other words, a spraying process typically cannot be used to realize a photoluminescent structure with a desirable conformal coating of phosphor powder. Furthermore, if spraying process is used to form a thin photoluminescent structure, a top portion of the photoluminescent structure right on top of an LED semiconductor die tends to have particle aggregation effect. This aggregated phosphor powder distribution will generate substantially large optical voids comprised of optically transparent resin material without much phosphor material mixed inside. This phosphor material aggregation effect is the main reason for phosphor-converted LEDs having undesirable “blue light spots”. The blue light leaking from the substantially large and substantially transparent voids may generate a locally strong intensity of blue light, which causes poor spatial color uniformity, and also poses the risk of blue-light hazard to human eyes. Other side effects due to phosphor powder aggregation include poor color uniformity, low manufacturing yield, and low phosphor wavelength conversion efficiency.
Another encountered issue when using CSP-type LED devices is the device reliability. One of the major technical features of a CSP-type LED device is that it does not possess a submount substrate or lead frame and directly exposes underneath bonding pads from an LED semiconductor die. Without a submount substrate, a photoluminescent structure of the CSP LED devices is exposed from underneath as well. A reflow soldering process is usually used when the CSP LED device is bonded onto an application substrate, such as a PCB. During the reflow soldering process, the soldering flux is typically utilized to improve the bonding quality between a CSP LED device and a substrate. However, the additives contained in the soldering flux tend to react with a benzene structure, which is widely found in silicone binder materials. However, a photoluminescent structure may be composed of a silicone resin material. This chemical reaction will form an adverse dark layer on the lower surface of a photoluminescent structure. The optical efficacy and reliability of the CSP LED device will suffer accordingly.
Therefore, providing a mass production solution is needed to address those aforementioned issues of CSP LED devices by providing improved interface adhesion strength between an LED semiconductor die and a photoluminescent structure, improving the spatial color uniformity and the CCT binning consistency, increasing the optical efficacy, and preventing the formation of an adverse dark layer on a lower surface of the photoluminescent structure.